The invention relates to isolation amplifiers and to modulation and demodulation techniques that may be useful in isolation amplifiers, including circuitry for converting voltage-to-frequency, phase, or pulse width representations thereof, and to circuitry for converting such representations back to a voltage or current signal representative of the original analog voltatge, and also including a high precision voltage-to-duty-cycle conversion technique and a low ripple, high bandwidth technique for converting such representations back to an analog signal representative of the original input voltage.
Voltage-to-duty-cycle conversion techniques and voltage-to-frequency conversion techniques are commonly used in various applications. Recovery of an analog input voltage is frequently accomplished by duty-cycle-to-voltage demodulators or frequency-to-voltage demoldulators. Examples of common applications of such circuits include switching power supplies, DC-to-DC converters, and isolation amplifiers. In the prior art, precision voltage-to-duty-cycle conversion has been preformed by comparing a linear triangular waveform voltage to an analog input voltage level, producing a "1" level when the triangular waveform voltage exceeds the analog input voltage, and producing a "0" level when the triangular waveform voltage is less than the analog input voltage. As the analog input voltage is increased, the duty cycle of the digital output waveform is decreased proportionally. The accuracy of the voltage-to-duty-cycle transfer function depends greatly upon the linearity of the triangular waveform and on the accuracy of the comparator used, and some of the information contained in the analog input voltage is lost. Also, the accuracy of such prior voltage-to-duty cycle conversion circuits is subject to input offset errors, i.e., errors in the relationship between a zero value of the analog input voltage and the corresponding 50 percent duty cycle of the duty-cycle-modulated (DCM) digital output waveform.
Charge balanced demodulators of the type that perform frequency-to-voltage conversion or duty-cycle-to-voltage conversion are well known. Such circuits receive a frequency modulated or duty-cycle encoded input pulse signal. This signal typically is applied to a high pass filter and applied to the input of a suitable demodulation interface circuit, which can be a one-shot circuit for a frequency-to-voltage converter or an edge triggered latch circuit for a duty-cycle-to-voltage converter. The output of the demodulation interface circuit controls a switch that couples a constant reference current to the inverting input of an operational amplifier, the non-inverting input of which is connected to ground. The output of the operational amplifier is connected through an integrating circuit including an integrating feedback capacitor and a parallel feedback resistor to the inverting input. The average current through the feedback capacitor must be zero, and the output voltage of the operational amplifier assumes a value necessary to cause the inverting input to be at a virtual ground voltage. The transfer characteristic of the output voltage to the modulated input signal frequency or dutycycle is very accurate, but the output voltage contains a large amount of ripple voltage. Although the amount of ripple voltage can be decreased by increasing the feedback capacitance, the bandwidth also is decreased. The trade-off between the bandwidth and ripple voltage is a significant limitation in many systems, such as in servo loops, wherein the ripple must be low but low bandwidth may cause loop instability.
Previous techniques used to minimize ripple voltage without affecting bandwidth have been less than satisfactory because they limit the range of carrier frequencies at which the demodulator can operate and/or they limit the accuracy of the transfer characteristic. For example, using a low pass filter in series with an output is not effective if a large operating frequency range is required. Furthermore, such filters are expensive. Another technique is to use the charge balanced demodulator as a feedback component in a phase locked loop, but this technique suffers from lack of versatility and is complex and expensive. Another technique has been to utilize a sample and hold circuit at the demodulator output and sample the output at a specific time during the demodulation cycle. This technique often adds greater error to the circuitry than already existed due to offsets and sample timing errors in the sample and hold circuit.
Isolation amplifiers are a common application of voltage-to-duty-cycle conversion circuits and charge balanced demodulators. The analog input voltage is converted to a digital signal, the duty-cycle of which represents the amplitude of the analog input voltage. This digital signal can be accurately transmitted over a standard isolation barrier, such as an optically coupled device or a transformer. After transmission over the isolation barrier, the input voltage signal is reconstructed by a duty-cycle to voltage demodulator. The accuracy of such isolation amplifiers has been limited by the above-mentioned inaccuracies of prior voltage-to-duty-cycle converters and prior charge balanced demodulators.
The state-of-the-art is such that there remains a substantial need for improved voltage-to-duty-cycle converters having higher accuracy than previously achievable and for charge balanced demodulators that are inexpensive, and also having higher speed, higher bandwidth, and lower ripple voltage than prior voltage-to-duty-cycle converters.
A problem of demodulators in circuits such as isolation amplifiers is that noise signals may be close to the modulator/demodulator carrier signal frequency. If this is the case, the modulator produces a difference signal of relatively low frequency, the carrier signal itself, and a sum signal having the frequency equal to the sum of the noise signal and the carrier signal. Ordinarily, it is quite easy to filter out the carrier signal and the sum signal, but difficult to filter out the difference signal (because its frequency is low) without also filtering the desired low frequency modulation signal.
Therefore, it would be desirable to provide an improved modulator and demodulator system that avoids the need to filter out noise difference signals produced by a modulator. It also would be desirable to provide an improved modulator and demodulator system which can function as a sharp filter for noise signals of known frequencies.
Another problem with prior isolation amplifiers is the difficulty of achieving a precise, transfer function that is relatively independent of temperature and variations in processing parameters. cSUMMARY OF THE INVENTION
It is an object of the invention to provide a voltage-to-duty-cycle conversion circuit and technique that is more accurate than the closest prior art devices.
It is another object of the invention to provide a voltage-to-duty-cycle converter circuit that is accurate despite non-linearities in a triangular waveform input voltage thereto.
It is another object of the invention to provide a charge balanced demodulator which has low ripple and high bandwidth without degrading the accuracy of the transfer characteristic.
It is another object of the invention to provide such a charge balanced demodulator with simple, inexpensive circuitry.
It is another object of the invention to provide a low cost, high performance isolation amplifier capable of undergoing very large, rapid changes in the difference between ground voltages on opposite sides of the isolation barrier without loss of information or data than is the case for the closest prior art isolation amplifiers.
It is another object of the invention to provide an isolation amplifier that avoids the need to filter out low frequency noise difference signals.
Briefly described, and in accordance with one embodiment thereof, the invention provides a modulator-to-modulator system including a modulator circuit including a first current switching circuit for producing a first current that is switched between positive and negative values in response to a duty-cycle-modulated signal produced by the modulator circuit in response to an analog input voltage, an isolation barrier for transmitting the duty-cycle-modulated signal to a demodulator circuit included in the isolation amplifier, the demodulator circuit including a second current switching circuit for producing a second current that is switched between positive and negative values in response to the duty-cycle-modulated signal transmitted across the isolation barrier. In one described embodiment of the invention, the demodulator is a charge balanced demodulator wherein a sample and hold circuit has an analog input coupled to an output of an integrating circuit that integrates the second current to produce an output of the sample and hold circuit and an analog output voltage that is accurately representative of the analog input voltage. The analog output voltage is coupled by a feedback resistor to the input of an operational amplifier included in the integrating circuit. A charge balance capacitor of the integrating circuit is connected between the output of the operational amplifier and an input thereof. In a described embodiment of the invention, the first and second current switching circuits are precisely matched to produce the same ratios of positive to negative values in response to the duty-cycle-modulated signal. Both the demodulator circuit and modulator circuit are fabricated on separate portions of a single large semiconductor chip, and trimmable components are precisely trimmed to produce the precise matching. The semiconductor chip then is cut into separate pieces to provide a modulator chip and a demodulator chip which are packaged in a single package and connected to terminals of small capacitors which constitute the isolation barrier. In one described embodiment, the modulator includes an integrating circuit which integrates the algebraic sum of the input current and the first current. The output of that integrating circuit is applied to one input of a hysteresis comparator, the output of which drives the first current switching circuit. A signal synchronized to a known noise signal is applied to the other input of the hysteresis comparator, causing the modulator carrier frequency to be synchronized to the noise signal. This prevents the modulator from generating a low frequency beat signal that would be difficult to filter. In another embodiment, the sample and hold circuit is implemented by a "bucket brigade" sample and hold circuit which includes two sample and hold circuits cascaded together to further reduce voltage ripple in the analog output voltage.